Imaging devices are useful in a large number of applications. Particularly in the field of devices operating in remote locations and/or used in connection with the sending of image data in real time or near real time, focal plane arrays including photon detectors capable of producing an electrical signal in response to sensing photons have been developed. Of the various technologies produced for use with such focal plane arrays, charge coupled devices (CCDs) have proven to provide exceptional optical performance. Therefore, high performance photon detectors often feature a CCD sensor.
Additionally, in the field of aerospace sensors, Time Delayed Integration Charge Coupled Devices (TDI CCDs) are uniquely suited to on orbit observations due to their nearly noiseless charge transfer and summing capability combined with an orbital motion that scans the image across the sensor.
CCDs, however, are characterized by relatively high power consumption. For devices in which high power consumption is a concern, devices formed using other processes, such as Complimentary Metal Oxide Semiconductor (CMOS) devices, are preferable. In particular, the technology required to produce low power consumption, densely packaged processing circuitry using CMOS technology is well developed. However, CMOS photon sensors typically produce a noisier signal than do CCD devices, and generally have less desirable optical performance. In addition, it is more difficult to perform the function of Time Delayed Integration (TDI) in a purely CMOS photon sensor.
Accordingly, it would be desirable to create a photon sensor in which the exceptional optical performance of a CCD photon detector was combined with the low power consumption and dense circuitry packaging available using CMOS processes. Combining the CCD and CMOS processes on a single substrate, however, has proven difficult. Such difficulties arise from fundamental incompatibilities between the two processes, including different processing temperatures and required oxide thicknesses. In addition, devices created on a single substrate using both CCD and CMOS technologies can suffer from poor image quality because of charge transfer inefficiency and high noise due to non-optimized processes.
In order to avoid integrating incompatible fabrication process technologies, devices that utilize a structure formed on a first substrate using a first process (e.g. a CCD photon detector) interconnected to a second substrate using a second process (e.g. a CMOS readout) have been developed. Such systems have typically sensed or read an amount of charge from the detector (first) substrate, and have then amplified the charge on the readout and processing (second) substrate to obtain a signal that can be used to create an image.
Additionally, it should be noted that charge detection generally can be performed only once with respect to a particular collection of charge. Also, providing an un-amplified or un-buffered signal from the CCD can in many cases result in a degraded signal.